text: When an ultra-short (100 femtoseconds or shorter) optical pulse illuminates a semiconductor and its wavelength (energy) is above the energy band-gap of the material, it photogenerates mobile carriers. Most carriers are generated near the surface of the material (typically within 1 micrometre) because pulses are absorbed exponentially with respect to depth. This has two main effects. Firstly, it generates a band bending that has the effect of accelerating carriers of different signs in opposite directions (normal to the surface), creating a dipole. This effect is known as surface field emission. Secondly, the presence of a surface creates a break of symmetry that causes carriers to move (on average) only into the bulk of the semiconductor. This phenomenon, combined with the difference of mobilities of electrons and holes, also produces a dipole. This is known as the photo-Dember effect and is particularly strong in high-mobility semiconductors such as indium arsenide.